Offshore oil drilling and production operations are conducted through a pipe, called a riser, running from a subsea wellhead to a surface platform or floating vessel. In order to support the weight of these risers and to control the stresses induced by ocean currents and vessel motions, the upper end of the riser is connected to a tensioning device. This riser tensioner maintains a predetermined range of tension throughout a range of vertical and lateral motions of the drilling or production rig.
The conventional approach to tensioning risers is to use a combination of a hydraulic or pneumatic mechanical cylinder, pressurized using a compressed gas, to apply the tensioning forces to the riser. Each riser tensioner is located on a deck of the floating platform or floating vessel and is structurally connected through its cylinders to the riser. The cylinders may be connected to the risers with wire rope or chain or directly connected through cylinder rods. The pressurized gas volume is typically contained in a separate pressure vessel referred to as an “accumulator”, positioned alongside the cylinder, which supplies sufficient gas volume to act as a gas spring. This combination of cylinder and accumulator acts to compress or expand the gas in response to vessel or riser movements, thereby maintaining a relatively uniform tension level in the riser.
For example, FIG. 1 illustrates a conventional wire riser tensioner system 100 including a double-acting hydraulic cylinder 110, a high-pressure accumulator 130, and a low-pressure accumulator 140. A piston 120 is disposed within an interior of the hydraulic cylinder 110 and configured to slide along an axial direction therein. The piston 120 includes a piston seat 122 and a piston extension 124. The piston seat 122 divides the interior of the cylinder 110 into a first variable-volume section 112 and a second variable-volume section 114. The volumes of the sections 112, 114 vary based on the position of the piston seat 122 within the cylinder 110. The piston extension 124 extends upwardly through the second section 114 of the cylinder 110. In certain implementations, the piston extension 124 may be hollow to reduce the weight of the piston 120.
A first sealing arrangement 127 is disposed at the piston seat 122 of the piston 120 to provide a seal between the first and section sections 112, 114 of the cylinder 110. A second sealing arrangement 129 is disposed between the piston extension 124 and an exterior of the cylinder 110 to seal the interior of the cylinder 110 as the piston 120 is slid therein. The second sealing arrangement 129 is located at an opposite end of the piston 120 from the first sealing arrangement 127.
The high-pressure accumulator 130 defines an interior 132 in which a first high pressure fluid (e.g., oil) may be stored. The high-pressure accumulator 130 is coupled to the cylinder 110 via a first flow path 150. The first flow path 150 provides a fluid pathway between the high-pressure accumulator 130 and the first variable volume section 112 of the cylinder 110. In certain implementations, the first flow path 150 extends between a bottom of the high-pressure accumulator 130 and a bottom of the cylinder 110. In certain implementations, a valve (e.g., an anti-recoil valve, a flow shut-off valve, etc.) 155 is disposed in the first flow path 150.
The high-pressure accumulator 130 also is configured to hold a second high-pressure fluid (e.g., compressed air, compressed nitrogen, or other gas). One or more air pressure vessels (APV's) 170 may be coupled to the high-pressure accumulator 130 via piping 175. Each APV 170 provides additional volume in which to store the second high-pressure fluid. In certain implementations, the APVs 170 are coupled to the high-pressure accumulator 130 using a ball-valve 172 or other valve arrangement. Providing additional volume in which the second high-pressure fluid may be contained aids in stabilizing the pressure of the second high-pressure fluid across the system 100.
The low-pressure accumulator 140 defines an interior 142 in which a low-pressure fluid (e.g., a lubricant) may be stored. The low-pressure accumulator 140 is coupled to the cylinder 110 via a second flow path 160. The second flow path 160 provides a fluid pathway between the low-pressure accumulator 140 and the second variable volume section 114 of the cylinder 110. For example, the second flow path 160 provides a fluid pathway between the low-pressure accumulator 140 and an annulus area around the piston extension 124. In certain implementations, the second flow path 160 extends between a top of the low-pressure accumulator 140 and a top of the cylinder 110. The low-pressure accumulator 140 is isolated from the high-pressure accumulator 130.
Accordingly, as the piston 120 is moved within the cylinder 110, the first high-pressure fluid is moved between the high-pressure accumulator 130 and the first variable-volume section 112 of the cylinder 110 through the first flow path 150. In addition, the low-pressure fluid is moved between the low-pressure accumulator 140 and the second variable volume section 114 of the cylinder 110 through the second flow path 160 as the piston 120 moves in the cylinder 110.